This invention relates to a closure for a container for the packaging of a vacuum packed product. More particularly, the invention relates to a vacuum indicating closure for a container for the packaging of vacuum packet product. Even more particularly, the invention relates to a closure which is formed from a thermoplastic material and which is of a vacuum indicating character when used on a container containing a vacuum packed product.
Many food products which are packaged in glass jars are packaged under a partial vacuum to prevent spoilage or to preserve flavor, and it is important that the closure for such a container be able to seal the container properly to maintain the vacuum in the container until the first opening thereof. It has also been recognized that it is desirable for a closure for a container for a vacuum packed product to incorporate means which will indicate the presence or absence of the desired degree of vacuum, and the prior art is familiar with metal closures which incorporate such a feature. For example, U.S. Pat. No. 4,093,094 (Smalley, et al.) describes a home canning closure in which the metal lid of a multi-piece closure incorporates a vacuum indicating button. The button in such a closure is deflected downwardly by the presence of a suitable degree of partial vacuum in the associated container, and because of the inherent elasticity of the closure, the deflected button will pop up to its normal position upon the release of the vacuum and the resulting repressurization of the container.
The use of a vacuum button or panel has heretofore been limited to metal closures or at least to closures with metal lids, however, and many of the plastic materials which have heretofore been widely utilized in the manufacture of container closures have not incorporated a similar vacuum indicating feature. The reason for this is that such plastic materials tend to be dimensionally stable only when they are under no appreciable load, and the distortion of a vacuum panel or button in the central panel of such a closure by virtue of the vacuum in the associated container would normally cause the central panel to gradually elongate (or creep) under the continued loading resulting from the vacuum. This problem could be aggravated in the case of a container exposed to abnormally high temperatures, such as, for example, those prevailing in non-air conditioned warehouses during summer months, where filled containers are somtimes stored for prolonged periods of time. In a case such as this a clear pop-up of a stressed vacuum indicating panel in a plastic closure cannot be assured.